Effect of toasting time on the browning of sliced bread

Slices of wheat bread were toasted for different times until a distinct intensity of brown colour was reached. Two assays were carried out: prolonged toasting times (5-60min) and reduced toasting times (0.5-5 min). The browning indicators (furosine, available lysine, hydroxymethylfurfural (HMF), colour and absorbance at 284 and 420nm) were determined. The precision of all indicators used was high (CV < 4%). No furosine or HMF was detected in the dough before baking. The furosine content increased until 7min (299 mg per 100 g protein) and then decreased to 2.9mg per 100 g protein at 60minutes. For the first toasting times (0.5, 1 and 2min) the furosine content decreased slightly. Available lysine reached losses of 50% after 25min of heating. The toasting of bread increased HMF values from 12 to 2025mg kg(-1) for the assay at prolonged times of heating and from 1.3 to 4.2mg kg(-1) at reduced times (0.5-5min). The HMF content decreased (1000mg kg(-1)) when the sliced bread was toasted until it burnt. Colour (E, 100 - L*) and absorbance at 284 and 420nm always increased. High linear correlations (r(2)>0.860) were obtained between browning indicators and time (A(284)/time, A(420)/time, 100 - L*/time and HMF/time). (C) 2001 Society of Chemical Industry.